An inertia force of a running vehicle is governed by a total weight of the vehicle including weights of passengers and luggage, and the vehicle starts coasting after returning an accelerator pedal to disable a prime mover such as an engine to generate driving force. If deceleration of the vehicle is not especially demanded during coasting of the vehicle, a power transmission between the prime mover and drive wheels can be cutoff by bringing an engagement device disposed therebetween into disengagement. The prime mover may also be disconnected from the drive wheels by bringing the engagement device into disengagement if power transmission therebetween is not required when the vehicle is stopped by depressing a brake pedal or locking a parking gear. In addition, if a torque of the prime mover such as the engine is not required to generate torque, fuel supply to the engine can be stopped irrespective of a driver's operation by S & S (i.e., Stop and Start) control to save fuel.
A transmission adapted to transmit power by a friction between a transmission member and an input rotary member or an output rotary member is known in the art, but the transmission member and the input and the output rotary members of the transmission may be damaged by a slippage of the transmission member. In order to reduce damage of the transmission, the conventional vehicles are configured to reduce such slippage of the transmission member by keeping a torque transmitting capacity of the engagement device to be smaller than that of the transmission. In this kind of vehicle, application of excessive torque to a contact face between the transmission member and the rotary member can be prevented by allowing the engagement device to slip when a large torque is applied to a powertrain. That is, in the vehicle thus structured, the engagement device serves as a fuse. The torque transmitting capacities of the engagement device and the transmission can be controlled by controlling hydraulic pressure in a hydraulic actuator thereof applied from an electric oil pump or a mechanical oil pump driven by a torque of the engine.
After the hydraulic pressures in the actuators of the engagement device and the transmission are lowered by the S & S control, if the engine is restarted by the S & S control and the oil is delivered simultaneously to the engagement device and the transmission, the torque transmitting capacity of the engagement device would exceed the torque transmitting capacity of the transmission in relation to a structural difference therebetween. In order to avoid such disadvantage, the control device taught by Japanese Patent Laid-Open No. 2001-082594 is configured to start engagement of the engagement device after a line pressure as an initial pressure of the actuator is raised to a level at which a slippage between a belt and a pulley of a continuously variable transmission will not be caused. That is, the torque transmitting capacity of the engagement device is increased after increasing the torque transmitting capacity of the transmission sufficiently. According to the teachings of Japanese Patent Laid-Open No. 2001-082594, torques applied to the engagement device and the transmission are reduced by keeping the engine to an idling speed until the engagement of the engagement device is completed.
However, if a shift position is shifted from neutral position to drive position immediately after restarting the engine so that the pressurized oil is delivered to the transmission and the engagement device after the hydraulic pressures in the belt-driven continuously variable transmission and the engagement device have been lowered by stopping the engine under the S & S control, the torque transmitting capacity of the engagement device would exceed the torque transmitting capacity of the transmission. In order to avoid such disadvantage, the control device taught by Japanese Patent Laid-Open No. 2011-133012 is configured to control hydraulic pressure applied to the engagement device based on a torque transmitting capacity of the belt-driven continuously variable transmission in such a manner that the torque transmitting capacity of the engagement device is kept to be smaller than that of the transmission, if a shifting operation is carried out immediately after restarting the engine.
In turn, Japanese Patent Laid-Open No. 2008-014362 describes the controller of continuously variable transmission configured to increase a belt clamping pressure in accordance with a road gradient when starting the vehicle on an upward slope.
However, if engagement of the engagement device is started after restarting the engine so that the torque transmitting capacity of the belt-driven continuously variable transmission has been increased as taught by Japanese Patent Laid-Open No. 2001-082594, power cannot be transmitted to the drive wheels until the engagement device is enabled to transmit torque and hence acceleration response is degraded. In addition, if the hydraulic pressure applied to the engagement device is determined based on the torque transmitting capacity of the transmission when delivering oil to the transmission in response to a shift operation after restarting the engine as taught by Japanese Patent Laid-Open No. 2011-133012, the control of the engagement device may be complicated.